Today is the day thousands are coming together in san francisco, all to fulfill one little boy's wish to become a superhero. Look out, san francisco. It's batman to the rescue. But it's not bruce wayne behind that mask. It's 5-year-old miles scott. This morning, the city by the bay is transforming into the dark knis playground, gotham city. The reason, this young caped crusader has been battling leukemia for three years. And he has one big dream. To be batman. He's my favorite superhero. Reporter: And thank to his friends at make-a-wish, that dream is m co-ing true. We started watching the old abam west batmans. He likes being the character. He likes putting on the costume and donning the mask. Reporter: Today, miles will nown simply as batkid. Chasing criminals in a bat mobile. Rescuing a damsel in distress on a cable car. He'll assist in catching the riddler and raid a downtown vault, all before getting the key to the city. He thinks he gets to be batkid for the day. I don't think he has any idea how big this thing got. Reporter: Thousands of volunteers have signed up to cheer on batkid, while he captures some notorious villains and saves the city. This morning's adventure will be a watershed moment for the scotts, as miles finished his treatment in june. It's a big party at the end of a long road. Reporter: He's now in remission and ready to take on the world. &#9834; Batman &#9834; Reporter: Taking on all of san francisco. And he doesn't know. His parents don't even know, how big this is going to be. It's all been kept a secret. We can't wait to see it unfold. If I were there, I would be his batgirl, robin. And good for you, san francisco. To come together like that for that kid. That's terrific.

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